Multi-piece solid golf ball

ABSTRACT

The invention provides a multi-piece solid golf ball having a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof. The ball is characterized both in that the core and the inner cover layer are formed so as to have a specific deflection ratio therebetween, and also by the combination of an inner cover layer formed to a specific material hardness and thickness, an outer cover layer formed to a specific material hardness, and dimples which satisfy specific conditions. This multi-piece solid golf ball is able to substantially reduce the distance traveled by the ball when struck at a high head speed, while at the same time holding down the decrease in distance when struck at a low head speed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 12/757,829 filed on Apr. 9, 2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball having a solid core, an inner cover layer and an outer cover layer, and having numerous dimples on a surface of the outer cover layer. More specifically, the invention relates to a multi-piece solid golf ball which substantially reduces the distance traveled by the ball when struck at a high head speed (head speed is sometimes abbreviated below as “HS”) while at the same time undergoing little reduction in distance when struck at a low HS.

With recent advances in golfing equipment such as balls and clubs, golf balls have come to travel increasing distances. For this reason, to keep play fair, strict rules have been adopted which establish, in the case of a golf club, for example, the size of the head and the length of the shaft. Similarly, restrictions have been placed on certain characteristics of a golf ball, such as its size, weight and initial velocity, so as to limit excessive ball travel of the sort that would result in a loss of fair play.

The distance traveled by a golf ball is generally held down by limiting the initial velocity. However, in such cases, both at high head speeds and low head speeds, the distance traveled is often reduced in about the same ratio. As a result, such balls have significant drawbacks for low HS players.

As another approach, various golf balls have been disclosed which, by optimizing the dimples on the surface of the ball, lower the flight trajectory and hold down decreases in distance.

For example, JP-A 05-103846 describes a golf ball in which the dimple diameter, dimple depth and number of dimples have been optimized. JP-A 10-043342 and JP-A 10-043343 disclose golf balls in which the amount of deformation by a ball when compressed under a load of 100 kgf has been optimized, along with which the dimple diameter divided by the dimple depth has been set to a value of from 10 to 15 or the dimple space volume as a proportion of the total volume of a hypothetical sphere were the surface of the ball to have no dimples thereon has been set to from 0.7 to 1.1%. JP-A 2000-107338 discloses a practice golf ball in which the ball weight and diameter have been optimized.

In addition, JP-A 6-218078, JP-A 6-343718, JP-A 7-24085, JP-A 9-239068, JP-A 10-151226, JP-A 10-201880, JP-A 11-104273, JP-A 11-104271, JP-A 2002-85588 and JP-A 2002-85589 disclose golf balls having a cover with a relatively hard inner layer and a relatively soft outer layer.

It is therefore an object of the present invention to provide a golf ball which can achieve a superior distance in a low HS range while holding down the distance traveled in a high HS range.

SUMMARY OF THE INVENTION

The inventors have conducted extensive investigations in order to achieve the above object. As a result, they have found that, in a multi-piece solid golf ball composed of a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof, by forming the core and the inner cover layer so as to have a specific deflection ratio therebetween; by specifying the thickness and material hardness (Shore D) of the inner cover layer, the material hardness (Shore D) of the outer cover layer, and also the size relationship between the material hardness of the inner cover layer and the material hardness of the outer cover layer; by specifying, for the dimples formed on the surface of the outer cover layer, the number of dimples, the dimple surface coverage (SR), the dimple volume ratio (VR), the dimple types, the average dimple depth, and the dimple diameter-to-depth ratio (DM/DP); and by maintaining the coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm at a given ratio or more of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm, synergistic effects arising from dimple optimization and the suitable hardness relationship between the inner cover layer and the outer cover layer make it possible to substantially reduce the distance traveled by the ball when struck at a high HS while at the same time holding down the decrease in distance when the ball is struck at a low HS.

That is, unlike conventional methods of lowering the ball initial velocity or core initial velocity, the golf ball of the present invention is able, by combining low-trajectory dimples with the internal structure (multilayer structure) of the ball, to substantially reduce the distance traveled by the ball when struck at a high HS while at the same time holding down to the extent possible, relative to the reduction in distance on high HS shots, the reduction in the distance traveled by the ball on low HS shots. As used herein, “distance” refers to the total distance traveled by a golf ball, including both the carry and the run.

Accordingly, the invention provides the following multi-piece solid golf balls.

[1] A multi-piece solid golf ball comprising a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof,

wherein the inner cover layer has a thickness of from 0.8 to 3.0 mm and a material hardness, in terms of Shore D hardness, of from 50 to 80, the outer cover layer has a material hardness, in terms of Shore D hardness, of from 35 to 60, and the material hardness of the inner cover layer is higher than the material hardness of the outer cover layer;

the ratio of deflection by a sphere composed of the solid core encased by the inner cover layer (inner cover layer-encased sphere) when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (inner cover layer-encased sphere deflection)/(solid core deflection), is from 0.73 to 0.84;

the dimples number at least 250 but not more than 500, have a surface coverage (SR) of at least 70% and a volume ratio (VR) of at least 1.0%, are of at least three types of mutually differing dimple diameter (DM) and/or dimple depth (DP), and have an average depth of at least about 0.18 mm and an average diameter-to-depth ratio (DM/DP) of not more than about 23;

and the ball has a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is maintained at 60% or more of a coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm.

[2] The multi-piece solid golf ball of [1] wherein, letting Da represent dimples having a diameter of at least 3.7 mm and Db represent dimples having a diameter of less than 3.7 mm, the ratio (total number of Db dimples)/(total number of Da dimples) is at least about 0.005 but not more than about 1. [3] The multi-piece solid golf ball of [2], wherein the dimples Da having a diameter of at least 3.7 mm account for at least about 75% of the total dimple volume. [4] The multi-piece solid golf ball of [1], wherein the outer cover layer has a thickness of from 0.5 to 3.0 mm. [5] The multi-piece solid golf ball of [1], wherein the value obtained by subtracting the material hardness of the outer cover layer from the material hardness of the inner cover layer (inner cover layer material hardness−outer cover layer material hardness) is, in terms of Shore D hardness, at least 5 but not more than 50. [6] The multi-piece solid golf ball of [1], wherein the dimples have an average edge angle of from 11 to 17 degrees. [7] The multi-piece solid golf ball of [1], wherein the proportion of dimples having an edge angle of from 12 to 16 degrees is more than 70% of the total number of dimples formed on the surface of the ball. [8] The multi-piece solid golf ball of [1], wherein the value obtained by subtracting the inner cover layer material hardness from a surface hardness (Hs) of the core (Hs—inner cover layer material hardness) is, in terms of Shore D hardness, at least −15 and not higher than +5. [9] The multi-piece solid golf ball of [1], wherein the value obtained by subtracting the outer cover layer material hardness from a surface hardness (Hs) of the core (Hs—outer cover layer material hardness) is, in terms of Shore D hardness, at least 0 and not higher than +20. [10] The multi-piece solid golf ball of [1], wherein the ratio of deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (ball deflection)/(solid core deflection), is from 0.67 to 0.76. [11] The multi-piece solid golf ball of [1], wherein the ratio of deflection by the ball when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) to deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (600 kgf deflection/130 kgf deflection), is from 3.3 to 3.8. [12] The multi-piece solid golf ball of [1], wherein the core has a center hardness (Hc), a surface hardness (Hs) and a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, in terms of Shore D hardnesses, satisfy the following conditions:

Hm−Hc=0 to 7,

Hs−Hm=11 to 25, and

Hs−Hc≧16.

[13] The multi-piece solid golf ball of [1], wherein the core has a center hardness (Hc), a surface hardness (Hs) and a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, in terms of Shore D hardnesses, satisfy the following condition:

Hs−Hm>(Hm−Hc)×3.

[14] The multi-piece solid golf ball of [1] wherein dimples Da with a diameter of at least 3.7 mm have an average diameter of at least 3.7 mm but not more than 6 mm, and dimples Db with a diameter of less than 3.7 mm have an average diameter of at least 1 mm but less than 3.7 mm. [15] The multi-piece solid golf ball of [1], wherein dimples Da with a diameter of at least 3.7 mm have an average depth of from 0.05 to 0.5 mm, and dimples Db with a diameter of less than 3.7 mm have an average depth of from 0.05 to 0.3 mm. [16] The multi-piece solid golf ball of [1], wherein dimples Da with a diameter of at least 3.7 mm have an average volume of from 0.8 to 3.0 mm³, and dimples Db with a diameter of less than 3.7 mm have an average volume of from 0.2 to 1.5 mm³. [17] The multi-piece solid golf ball of [1], wherein dimples Da with a diameter of at least 3.7 mm have an average diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 7 to 25, and dimples Db with a diameter of less than 3.7 mm have an average diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 10 to 30. [18] The multi-piece solid golf ball of [1], wherein the cover is formed of a material comprising:

(A) a thermoplastic polyurethane material, and

(B) an isocyanate mixture obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate.

[19] The multi-piece solid golf ball of [1], wherein the cover is formed of a material comprising:

(D) a thermoplastic polyurethane, and

(E) a polyisocyanate compound.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing the internal structure of a multi-piece solid golf ball according to an embodiment of the present invention.

FIG. 2 is a schematic view illustrating a dimple used in the present invention.

FIG. 3 is a top view of a golf ball showing a dimple pattern (I) used on a ball in an example of the invention.

FIG. 4 is a top view of a golf ball showing a dimple pattern (II) used on a ball in an example of the invention.

FIG. 5 is a front view of a golf ball showing a dimple pattern (III) used on a ball in a comparative example.

FIG. 6 is a front view of a golf ball showing a dimple pattern (IV) used on a ball in a comparative example.

FIG. 7 is a front view of a golf ball showing a dimple pattern (V) used on a ball in a comparative example.

FIG. 8 is a front view of a golf ball showing a dimple pattern (VI) used on a ball in a comparative example.

FIG. 9 is a cross-sectional view for explaining the edge angle of a dimple.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention is a multi-piece solid golf ball having a solid core (sometimes referred to below as simply the “core”), an inner cover layer and an outer cover layer. The outer cover layer has a surface with numerous dimples formed thereon. By forming the core and the inner cover layer so as to have a specific deflection ratio therebetween, and by combining an inner cover layer formed to a specific material hardness and thickness, an outer cover layer formed to a specific material hardness and dimples which satisfy the subsequently described specific parameters, the distance traveled by the ball on shots at a high HS can be substantially reduced while suppressing a decrease in the distance traveled by the ball on shots at a low HS. As used in the present invention, “high HS range” refers to a range of about 50 to 60 m/s and “low HS range” refers to a range of about 30 to 40 m/s.

The internal structure of the inventive golf ball G is described. Referring to FIG. 1, the ball G has a three-layer construction composed of at least a core 1, an inner cover layer 2 encasing the core 1, and an outer cover layer 3 encasing the inner cover layer 2. In this invention, the inner cover layer 2 and the outer cover layer 3 are sometimes referred to collectively as the “cover.” Numerous dimples D are formed on the surface of the outer cover layer 3; these dimples D satisfy the specific parameters of the invention. It should be noted that, although FIG. 1 shows a three-layer construction arrived at by forming a core 1, an inner cover layer 2 and an outer cover layer 3, any of these layers may be optionally formed as a plurality of two or more layers without departing from the scope of the invention. For example, the core may be formed as a plurality of layers.

The core in the invention may be formed using a rubber composition containing, for example, a base rubber and also such ingredients as a co-crosslinking agent, an organic peroxide, an inert filler, sulfur and an organosulfur compound. The base rubber of the rubber composition is preferably one composed primarily of a known polybutadiene.

In the present invention, an organosulfur compound may be blended with the base rubber in order to increase the rebound of the core. When an organosulfur compound is included, the amount included per 100 parts by weight of the base rubber may be set to preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight. The upper limit in the amount included may be set to preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight by weight, and even more preferably not more than 2 parts by weight. If the amount of organosulfur compound included is too small, a sufficient core rebound-increasing effect may not be obtained. On the other hand, if too much organosulfur compound is included, the core may become too soft, resulting in a poor feel when the ball is played and a poor durability to cracking on repeated impact.

The diameter of the core, although not subject to any particular limitation, may be set to from 30 to 40 mm. The lower limit value is preferably at least 32 mm, more preferably at least 34 mm, and even more preferably at least 35 mm. The upper limit value may be set to preferably not more than 39.5, more preferably not more than 39 mm, and even more preferably not more than 38.5 mm.

The core has a center hardness (Hc) which, although not particularly limited, may be set to, in terms of Shore D hardness, preferably at least 25, more preferably at least 28, and even more preferably at least 32. The upper limit also is not particularly limited and may be set to, in terms of Shore D hardness, preferably not more than 50, more preferably not more than 45, and even more preferably not more than 40. If the center hardness is too low, the rebound may be too low, resulting in a less than desirable distance, the feel at impact may be too soft, or the durability of the ball to cracking on repeated impact may worsen. On the other hand, if the center hardness is too high, the spin rate may rise excessively, possibly resulting in a less than desirable distance, or the feel at impact may be too hard.

The core has a surface hardness (Hs) which, although not particularly limited, may be set to, in terms of Shore D hardness, preferably at least 45, more preferably at least 50, and even more preferably at least 53. The upper limit also is not particularly limited and may be set to, in terms of Shore D hardness, preferably not more than 70, more preferably not more than 65, and even more preferably not more than 60. If the surface hardness is too low, the rebound may be too low, resulting in a less than desirable distance, the feel at impact may be too soft, or the durability of the ball to cracking on repeated impact may worsen. On the other hand, if the surface hardness is too high, the feel at impact may be too hard or the durability to cracking on repeated impact may worsen.

The core has a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, although not particularly limited, may be set to, in terms of Shore D hardness, preferably at least 30, more preferably at least 33, and even more preferably at least 36. The upper limit also is not particularly limited and may be set to, in terms of Shore D hardness, preferably not more than 55, more preferably not more than 50, and even more preferably not more than 45. If the cross-sectional hardness is too low, the rebound may be too low, resulting in a less than desirable distance, or the durability to cracking on repeated impact may worsen. On the other hand, if the cross-sectional hardness is too high, the spin rate may rise excessively, resulting in a less than desirable distance, or the feel at impact may be too hard.

As used herein, “center hardness (Hc)” refers to the hardness measured at the center of the cross-section obtained by cutting the core in half (through the center), and “surface hardness (Hs)” refers to the hardness measured at the surface of the core (spherical surface). In addition, “cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface” refers to the hardness measured at a point midway between the core center and the core surface on the above cross-section. Also, “Shore D hardness” refers to the hardness measured using a type D durometer in general accordance with ASTM D2240-95.

In this invention, the value Hm−Hc obtained by subtracting the core center hardness (Hc) from the cross-sectional hardness (Hm) at an intermediate position between the core center and core surface, although not particularly limited, is preferably set to, in terms of Shore D hardness, from 0 to 7. The upper limit in this value may be set to, in terms of Shore D hardness, more preferably not more than 6, and even more preferably not more than 5. The lower limit may be set to, in terms of Shore D hardness, more preferably at least 2, and even more preferably at least 3. If the above value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value is too small, the spin rate may rise excessively, as a result of which the distance may be less than satisfactory.

The value Hs−Hm obtained by subtracting the cross-sectional hardness (Hm) at an intermediate position between the core center and core surface from the core surface hardness (Hs), although not particularly limited, is preferably set to, in terms of Shore D hardness, from 11 to 25. The upper limit in this value may be set to, in terms of Shore D hardness, more preferably not more than 22, and even more preferably not more than 20. The lower limit may be set to, in terms of Shore D hardness, more preferably at least 12, and even more preferably at least 15. If the above value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value is too small, the spin rate may rise excessively, as a result of which the distance may be less than satisfactory.

The value Hs−Hc obtained by subtracting the core center hardness (Hc) from the core surface hardness (Hs), although not particularly limited, may be set to, in terms of Shore D hardness, preferably at least 16, and more preferably at least 20. The upper limit in this value, although not particularly limited, may be set to, in terms of Shore D hardness, preferably not more than 40, and more preferably not more than 30. If this value is too small, the spin rate may rise excessively, as a result of which the distance may be less than satisfactory.

In addition, the core center hardness (Hc), the core surface hardness (Hs) and the cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface, although not particularly limited, preferably satisfy, in terms of Shore D hardnesses, the following condition:

Hs−Hm>(Hm−Hc)×3.

In cases where the hardness relationship among the various parts of the core departs from the above relationship, a good distance may not be achieved at both high head speeds and low head speeds.

The core deflection, i.e., the amount of deflection by the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), while not subject to any particular limitation, may be set within a range of from 2.0 to 6.0 mm. In this case, the lower limit value is preferably at least 2.5 mm, more preferably at least 2.8 mm, and even more preferably at least 3.2 mm. The upper limit value may be set to preferably not more than 5.5 mm, more preferably not more than 5.0 mm, and even more preferably not more than 4.5 mm. If the core is too much harder than the above range (small deflection), the spin will rise excessively, which is unsuitable for the dimples of the present invention. On the other hand, if the core is too much softer than the above range (large deflection), the feel of the ball at impact may become too soft and the durability to cracking on repeated impact may worsen.

The specific gravity of the core, while not subject to any particular limitation, may be set within a range of from 0.9 to 1.4. In such a case, the lower limit value is preferably at least 1.0, and more preferably at least 1.1. The upper limit value may be set to preferably not more than 1.3, and more preferably not more than 1.2.

In this invention, by using the above material to form the solid core 1, a golf ball capable of achieving a stable trajectory can be provided.

In the golf ball G of the invention, an inner cover layer 2 and an outer cover layer 3 are formed over the above solid core 1. In this invention, the material hardnesses and thicknesses of each of these layers are set as described below. Here, “material hardness” refers to the hardness (Shore D) of a sheet of the cover material that has been molded under applied pressure to a thickness of about 2 mm, as measured using a type D durometer in general accordance with ASTM D2240.

First, the material hardness of the inner cover layer is set to, in terms of Shore D hardness, at least 50, and may be set to preferably at least 55, and more preferably at least 60. The upper limit is set to, in terms of Shore D hardness, not more than 80, and is recommended to be preferably not more than 70, and more preferably not more than 65. When the material hardness of the inner cover layer is too high, the ball may have a poor feel at impact or a poor durability.

The thickness of the inner cover layer is set to at least 0.8 mm, and may be set to preferably at least 1.0 mm, more preferably at least 1.2 mm, and even more preferably at least 1.5 mm. The upper limit is not more than 3.0 mm, and is recommended to be preferably not more than 2.5 mm, more preferably not more than 2.0 mm, and most preferably not more than 1.8 mm. If the inner cover layer is too thin, the durability will worsen; on the other hand, if it is too thick, the ball will have a poor feel on impact.

The deflection by a sphere composed of the above core encased by the inner cover layer (inner cover layer-encased sphere), when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), although not subject to any particular limitation, may be set in the range of 2.0 to 5.5 mm. In this case, the lower limit is preferably at least 2.2 mm, more preferably at least 2.5 mm, and even more preferably at least 2.8 mm. The upper limit may be set to preferably not more than 5.0 mm, more preferably not more than 4.5 mm, and even more preferably not more than 4.0 mm. If the deflection is too small, the feel at impact may be too hard. On the other hand, if the deflection is too large, the feel at impact may be too soft and the durability to cracking may be poor.

The material hardness of the outer cover layer is set to, in terms of Shore D hardness, at least 35, and may be set to preferably at least 40, and more preferably at least 45. The upper limit is not more than 60, and is recommended to be preferably not more than 55, and more preferably not more than 53. If the material hardness of the outer cover layer is too high, the durability will worsen or the feel at impact will worsen.

The thickness of the outer cover layer, although not subject to any particular limitation, may be set to at least 0.5 mm, preferably at least 0.6 mm, and more preferably at least 0.7 mm. It is recommended that the upper limit be not more than 3.0 mm, preferably not more than 2.0 mm, more preferably not more than 1.5 mm, and even preferably not more than 1.2 mm. If the outer cover layer is too thin, a good feel at impact may not be obtained.

Moreover, in the present invention, the ball is formed in such a way that the material hardness of the inner cover layer is higher than the material hardness of the outer cover layer. In this case, the difference in hardness between the inner cover layer and the outer cover layer (inner cover layer material hardness−outer cover layer material hardness), although not subject to any particular limitation, may be set to, in terms of Shore D hardness, preferably at least 5, more preferably at least 8, even more preferably at least 10, and most preferably at least 12. It is recommended that the upper limit in this hardness difference be, in terms of Shore D hardness, preferably not more than 50, more preferably not more than 40, even more preferably not more than 30, and most preferably not more than 20. If the hardness difference is too small, the feel at impact may worsen; on the other hand, if it is too large, the durability may worsen.

The ball having the above outer cover layer formed therein has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) which, although not particularly limited, may be set in the range of 2.0 to 5.0 mm. The lower limit in this case is preferably at least 2.2 mm, more preferably at least 2.3 mm, and even more preferably at least 2.4 mm. The upper limit may be set to preferably not more than 4.5 mm, more preferably not more than 4.0 mm, and even more preferably not more than 3.5 mm. If the deflection is too small, the feel at impact may be too hard. On the other hand, if the deflection is too large, the feel at impact may be too soft or the durability to cracking may be poor.

Moreover, the ball has a deflection when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) which, although not particularly limited, may be set in the range of 7.0 to 14.0 mm. The lower limit in this case is preferably at least 8.0 mm, more preferably at least 8.5 mm, and even more preferably at least 9.0 mm. The upper limit may be set to preferably not more than 13.0 mm, more preferably not more than 12.0 mm, and even more preferably not more than 11.0 mm. If the deflection is too small, the feel at impact may be too hard. On the other hand, if the deflection is too large, the feel at impact may be too soft or the durability to cracking may be poor.

The inner cover layer and the outer cover layer preferably satisfy the following conditions in their relationship with the solid core.

The value obtained by subtracting the material hardness of the inner cover layer from the core surface hardness (Hs), which value is expressed as (Hs—inner cover layer material hardness), although not particularly limited, is preferably set to, in terms of Shore D hardness, at least −15 and not more than +5. The upper limit, in terms of Shore D hardness, may be set to more preferably not more than +3, and even more preferably not more than +1. The lower limit, in terms of Shore D hardness, may be set to more preferably at least −12, and even more preferably at least −8. If this value is too large, the spin rate of the ball may become too high. On the other hand, if this value is too small, the feel at impact may harden or the durability to cracking on repeated impact may worsen.

The value obtained by subtracting the material hardness of the outer cover layer from the core surface hardness (Hs), which value is expressed as (Hs—outer cover layer material hardness), although not particularly limited, is preferably set to, in terms of Shore D hardness, at least 0 and not more than +20. The upper limit, in terms of Shore D hardness, may be set to more preferably not more than 16, and even more preferably not more than 12. The lower limit, in terms of Shore D hardness, may be set to more preferably at least 4, and even more preferably at least 8. If this value is too large, the spin rate of the ball may become too high. On the other hand, if this value is too small, the durability to cracking on repeated impact may worsen.

In this invention, the ratio of deflection by a sphere composed of the solid core encased by the inner cover layer (inner cover layer-encased sphere) when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (inner cover layer-encased sphere deflection)/(solid core deflection), is from 0.73 to 0.84. The lower limit in this deflection ratio is preferably at least 0.75, and more preferably at least 0.77. The upper limit in this deflection ratio is preferably not more than 0.82, and more preferably not more than 0.80. If this deflection ratio is too large, the ball rebound will be inadequate or the spin rate will become too high. On the other hand, if this value is too small, the feel at impact will harden or the durability to cracking on repeated impact will worsen.

In addition, although not particularly limited, the ratio of deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (ball deflection)/(solid core deflection), is preferably from 0.67 to 0.76. The lower limit in this deflection ratio is more preferably at least 0.69, and the upper limit is preferably not more than 0.74. If this deflection ratio is too large, the ball rebound may be inadequate or the spin rate may become too high. On the other hand, if this value is too small, the feel at impact may harden or the durability to cracking on repeated impact may worsen.

Moreover, although not particularly limited, the ratio of deflection by the ball when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) to deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (600 kgf deflection/130 kgf deflection), is preferably from 3.3 to 3.8. The lower limit in this deflection ratio is more preferably at least 3.4, and the upper limit is preferably not more than 3.7. If this deflection ratio is too large, the durability to cracking under repeated impact may worsen. On the other hand, if this value is too small, the spin rate may become high and the distance traveled by the ball may decrease regardless of the head speed at which the ball is struck.

The cover having the above construction may be formed of a known material exemplified by thermoplastic resins such as ionomeric resins, and various types of thermoplastic elastomers. Examples of thermoplastic elastomers include polyester-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

In the present invention, such cover materials are not subject to any particular limitation, although preferred use may be made of a cover material composed primarily of a material selected from the group consisting of the polyurethane materials (I), polyurethane materials (II) and ionomeric resin materials shown below. These materials, and molding methods for the same, are described in order below.

Polyurethane Material (I)

This material (I) is composed primarily of components A and B below:

(A) a thermoplastic polyurethane material, (B) an isocyanate mixture obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate.

Golf balls in which the cover has been formed of this material (I) can be endowed with an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact.

Next, each of the above components is described.

The thermoplastic polyurethane material (A) has a structure which includes soft segments made of a polymeric polyol (polymeric glycol), and hard segments made of a chain extender and a diisocyanate. Here, the polymeric polyol used as a starting material is not subject to any particular limitation, and may be any that is used in the prior art relating to thermoplastic polyurethane materials, such as polyester polyols and polyether polyols. Polyether polyols are preferable to polyester polyols because they enable the synthesis of thermoplastic polyurethane materials having a high rebound resilience and excellent low-temperature properties. Illustrative examples of polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred from the standpoint of the rebound resilience and low-temperature properties. The polymeric polyol has an average molecular weight of preferably from 1,000 to 5,000. A molecular weight of from 2,000 to 4,000 is especially preferred for synthesizing thermoplastic polyurethane materials having a high rebound resilience.

The chain extender employed is preferably one which is used in the art relating to conventional thermoplastic polyurethane materials. Illustrative, non-limiting, examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which is used in the art relating to conventional thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, for stable reactivity with the subsequently described isocyanate mixture (B), it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.

A commercial product may be advantageously used as the thermoplastic polyurethane material composed of the above-described material. Illustrative examples include those available under the trade names Pandex T-8290, Pandex T-8295 and Pandex T8260 (DIC Bayer Polymer, Ltd.), and those available under the trade names Resamine 2593 and Resamine 2597 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

The isocyanate mixture (B) is obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate. Here, the isocyanate compound (B-1) is preferably an isocyanate compound used in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. From the standpoint of reactivity and work safety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (B-2) is preferably a resin having a low water absorption and excellent compatibility with thermoplastic polyurethane materials. Illustrative examples of such resins include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins, and polyester elastomers (e.g., polyether-ester block copolymers, polyester-ester block copolymers). From the standpoint of the rebound resilience and strength, the use of a polyester elastomer, particularly a polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relative proportions of the thermoplastic resin (B-2) and the isocyanate compound (B-1), expressed as the weight ratio (B-2):(B-1), to be from 100:5 to 100:100, and especially from 100:10 to 100:40. If the amount of the isocyanate compound (B-1) relative to the amount of the thermoplastic resin (B-2) is too small, a greater amount of the isocyanate mixture (B) will have to be added in order to achieve an amount of addition sufficient for the crosslinking reaction with the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (B-2) will exert a large influence, rendering the physical properties of the material inadequate. On the other hand, if the amount of the isocyanate compound (B-1) relative to the amount of the thermoplastic resin (B-2) is too large, the isocyanate compound (B-1) may cause slippage to occur during mixing, making preparation of the isocyanate mixture (B) difficult.

The isocyanate mixture (B) may be obtained by, for example, adding the isocyanate compound (B-1) to the thermoplastic resin (B-2) and thoroughly working together these components at a temperature of from 130 to 250° C. using mixing rolls or a Banbury mixer, then either pelletizing or cooling and subsequently grinding. A commercial product such as that available under the trade name Crossnate EM30 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as the isocyanate mixture (B).

The above material (I) is composed primarily of the thermoplastic polyurethane material (A) and the isocyanate mixture (B) described above. In this material (I), the isocyanate mixture (B) is included in an amount, per 100 parts by weight of the thermoplastic polyurethane material (A), of at least 1 part by weight, preferably at least 5 parts by weight, and more preferably at least 10 parts by weight, but not more than 100 parts by weight, preferably not more than 50 parts by weight, and more preferably not more than 30 parts by weight. If too little isocyanate mixture (B) is included relative to the thermoplastic polyurethane material (A), a sufficient crosslinking effect will not be achieved. On the other hand, if too much is included, this may lead to discoloration of the molded material by unreacted isocyanate, which is undesirable.

In addition to above components (A) and (B), another component (C), although not essential, may also be included in the material (I). This other component is exemplified by thermoplastic polymeric materials other than thermoplastic polyurethane materials; illustrative examples include polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, polyethylene, and nylon resins. When component (C) is included, the amount is not subject to any particular limitation and may be suitably selected as appropriate for such purposes as adjusting the hardness, improving the resilience, improving the flow properties, and improving the adhesion of the cover material. The amount of component (C) included per 100 parts by weight of component (A) is set to preferably at least 10 parts by weight, and the upper limit is set to not more than 100 parts by weight, preferably not more than 75 parts by weight, and more preferably not more than 50 parts by weight. If necessary, various additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may also be suitably included in the above material (I).

Formation of the cover using the above material (I) may be carried out by a known molding method. For example, the cover may be molded by adding the isocyanate mixture (B) to the thermoplastic polyurethane material (A) and dry mixing, feeding the resulting mixture to an injection molding machine, and injecting the molten resin blend over the core. In such a case, the molding temperature varies with the type of thermoplastic polyurethane material (A), although molding is generally carried out within the temperature range of 150 to 250° C.

Reactions and crosslinking which take place in the golf ball cover obtained as described above are believed to involve the reaction of isocyanate groups with hydroxyl groups remaining in the thermoplastic polyurethane material to form urethane bonds, or the creation of an allophanate or biuret crosslinked form via a reaction involving the addition of isocyanate groups to urethane groups in the thermoplastic polyurethane material. Although the crosslinking reactions have not yet proceeded to a sufficient degree immediately after injection molding of the material (I), the crosslinking reactions can be made to proceed further by carrying out an annealing step after molding, in this way maintaining characteristics which are useful for a golf ball cover. “Annealing,” as used herein, refers to heat aging the cover at a constant temperature for a fixed length of time, or aging the cover for a fixed period at room temperature.

Polyurethane Material (II)

This material (II) is a single resin blend in which the primary components are (D) a thermoplastic polyurethane and (E) a polyisocyanate compound. By forming a cover composed primarily of such a polyurethane material (II), it is possible to achieve an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact without a loss of resilience.

As used herein, reference to a “single” resin blend means that the resin blend is not fed as a plurality of types of pellets, but rather is supplied to, for example, an injection molding machine as one type of pellet prepared by incorporating a plurality of ingredients into individual pellets.

To fully and effectively achieve the objects of the invention, a necessary and sufficient amount of unreacted isocyanate groups should be present within the cover resin material. Specifically, it is recommended that the combined weight of above components (D) and (E) account for at least 60%, and more preferably at least 70%, of the total weight of the cover. Components (D) and (E) are described in detail below.

The above thermoplastic polyurethane (D) is described. The thermoplastic polyurethane structure includes soft segments made of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments made of a chain extender and a polyisocyanate compound. Here, the long-chain polyol used as a starting material is not subject to any particular limitation, and may be any that has hitherto been used in the art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof. Of the long-chain polyols mentioned here, polyether polyols are preferred because they enable the synthesis of thermoplastic polyurethanes having a high rebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of cyclic ethers. These polyether polyols may be used singly or as a combination of two or more thereof. In the present invention, poly(tetramethylene glycol) and poly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-average molecular weight in the range of 1,500 to 5,000. By using a long-chain polyol having a number-average molecular weight within this range, golf balls made with a thermoplastic polyurethane composition having excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in the range of 1,700 to 4,000, and even more preferably in the range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chain polyol” refers to the number-average molecular weight calculated based on the hydroxyl number measured in accordance with JIS K-1557.

Any chain extender employed in the prior art relating to thermoplastic polyurethane materials may be advantageously used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. In the present invention, an aliphatic diol having 2 to 12 carbons is preferred, and 1,4-butylene glycol is more preferred.

Any polyisocyanate compound employed in the prior art relating to thermoplastic polyurethane materials may be advantageously used without particular limitation as the polyisocyanate compound. For example, use may be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving as above component D to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be a polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the aromatic diisocyanate to be 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction can be adjusted within a desirable range so as to make it possible to obtain a golf ball which is composed of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing component (D). Production may be carried out by either a prepolymer process or a one-shot process in which the long-chain polyol, chain extender and polyisocyanate compound are used and a known urethane-forming reaction is effected. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

A commercial product may be used as component (D). Illustrative examples include products available under the trade names Pandex T8295, Pandex T8290 and Pandex T8260 (DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component E, it is essential that, in at least some portion thereof within a single resin blend, all the isocyanate groups on the molecule remain in an unreacted state. That is, polyisocyanate compound in which all the isocyanate groups on the molecule are in a completely free state should be present within a single resin blend, and such a polyisocyanate compound may be present together with a polyisocyanate compound in which a portion of the isocyanate groups on the molecule are in a free state.

Various isocyanates may be used without particular limitation as the polyisocyanate compound. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, using 4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferred for achieving a good balance between the influence on moldability by, for example, the rise in viscosity associated with reaction with the thermoplastic polyurethane serving as component D, and the properties of the resulting golf ball cover material.

In Polyurethane Material (II), although not an essential ingredient, a thermoplastic elastomer other than the above thermoplastic polyurethane may be included as component F in addition to above components D and E. Including this component F in the above resin blend enables the flow properties of the resin blend to be further improved and enables various properties required of golf ball cover materials, such as resilience and scuff resistance, to be enhanced.

This component F, which is a thermoplastic elastomer other than the above thermoplastic polyurethane, is exemplified by one or more thermoplastic elastomer selected from among polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. The use of polyester elastomers, polyamide elastomers and polyacetals is especially preferred because the resilience and scuff resistance are enhanced, owing to reactions with isocyanate groups, while at the same time a good manufacturability is retained.

The relative proportions of above components D, E and F are not subject to any particular limitation. However, to fully achieve the advantageous effects of the invention, it is preferable for the weight ratio among the respective components to be (D):(E):(F)=100:2 to 50:0 to 50, and more preferably (D):(E):(F)=100:2 to 30:8 to 50.

In this invention, a single resin blend for forming the cover is prepared by mixing together component D, component E, and also optional component F. At this time, it is essential to select the mixing conditions such that, of the polyisocyanate compound, at least some polyisocyanate compound is present in which all the isocyanate groups on the molecule remain in an unreacted state. For example, treatment such as mixture in an inert gas (e.g., nitrogen) or in a vacuum state must be furnished. The resin blend is then injection-molded around a core which has been placed in a mold. To smoothly and easily handle the resin blend, it is preferable for the blend to be formed into pellets having a length of 1 to 10 mm and a diameter of 0.5 to 5 mm. Sufficient isocyanate groups in an unreacted state remain in these resin pellets; the unreacted isocyanate groups react with component D or component F to form a crosslinked material while the resin blend is being injection-molded about the core, or due to post-treatment such as annealing thereafter.

In addition, various optional additives may also be included in this cover-forming resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers, and parting agents may be suitably included.

The melt mass flow rate (MFR) of this resin blend at 210° C. is not subject to any particular limitation. However, to increase the flow properties and manufacturability, the MFR is preferably at least 5 g/10 min, and more preferably at least 6 g/10 min. If the melt mass flow rate of the resin blend is too low, the flow properties will decrease, which may cause eccentricity during injection molding and may also lower the degree of freedom in the thickness of the cover that can be molded. The melt mass flow rate is a measured value obtained in accordance with JIS-K7210 (1999 edition).

The method of molding the cover may involve feeding the above resin blend to an injection-molding machine and injecting the molten resin blend around the core. Although the molding temperature in this case will vary depending on the type of thermoplastic polyurethane, the molding temperature is generally from 150 to 250° C.

When injection molding is carried out, it is desirable, though not essential, to carry out such molding in a low-humidity environment by subjecting some or all places on the resin paths from the resin feed area to the mold interior to purging with an inert gas such as nitrogen or a low-moisture gas such as low dew-point dry air, or to vacuum treatment. Preferred, non-limiting, examples of the medium used for transporting the resin under applied pressure include low-moisture gases such as low dew-point dry air or nitrogen gas. By carrying out molding in such a low-humidity environment, the progression of reactions by isocyanate groups before the resin blend is charged into the mold interior is suppressed. By thus including, within the molded resin material, polyisocyanate in which some isocyanate groups are present in an unreacted state, it is possible to reduce variable factors such as an undesirable rise in viscosity and to increase the real crosslinking efficiency.

Techniques that may be used to confirm the presence of polyisocyanate compound in an unreacted state within the resin blend prior to injection molding about the core include those which involve extraction with a suitable solvent that selectively dissolves out only the polyisocyanate compound. An example of a simple and convenient method is one in which confirmation is carried out by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) measurement in an inert atmosphere. For example, when the above-described single resin blend (Polyurethane Material (II)) is heated in a nitrogen atmosphere at a temperature ramp-up rate of 10° C./min, a gradual drop in the weight of diphenylmethane diisocyanate can be observed from about 150° C. On the other hand, in a resin sample in which the reaction between the thermoplastic polyurethane material and the isocyanate mixture has been carried out to completion, a weight drop is not observed from about 150° C., but a weight drop can be observed from about 230 to 240° C.

After the above Polyurethane Material (II) has been injection-molded to form a cover, the properties as a golf ball cover can be additionally improved by carrying out annealing so as to induce the crosslinking reaction to proceed further. “Annealing,” as used herein, refers to aging the cover in a fixed environment for a fixed length of time.

Ionomeric Resin Material

In the present invention, “ionomeric resin material” refers to a resin composition which includes:

100 parts by weight of a resin component composed of

a base resin containing (a) from 95 to 50 wt % of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer and/or a metal salt thereof and

(b) from 0 to 20 wt % of an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof, and (c) from 0 to 50 wt % of a thermoplastic block copolymer composed of a crystalline polyolefin block and a polyethylene/butylene random copolymer; (d) from 5 to 170 parts by weight of a fatty acid or fatty acid derivative having a molecular weight of 280 to 1,500; and (e) from 0.1 to 10 parts by weight of a basic inorganic metal compound capable of neutralizing acid groups in components (a) and (d), and, if necessary, component (b).

Components (a) to (e) are described below.

Component (a) and component (b) serve as the base resin of the above resin composition. Component (a) is an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer and/or a metal salt thereof, and component (b) is an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof. In the present invention, either of above components (a) and (b) may be used singly or both may used in combination.

Here, above component (a) has a weight-average molecular weight (Mw) of preferably at least 100,000, more preferably at least 110,000, and even more preferably at least 120,000, but preferably not more than 200,000, more preferably not more than 190,000, and even more preferably not more than 170,000. The weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio for the copolymer is preferably at least 3, and more preferably at least 4, with the upper limit being preferably not more than 7, and more preferably not more than 6.5.

The olefin in component (a) generally has a number of carbons that is at least 2, but not more than 8, and preferably not more than 6. Illustrative examples of such olefins include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester may be, for example, a lower alkyl ester of an unsaturated carboxylic acid. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, isobutyl acrylate) is especially preferred.

The random copolymer serving as component (a) may be obtained by the random copolymerization of the above ingredients in accordance with a known method. Here, the unsaturated carboxylic acid content (acid content) within the random copolymer, although not subject to any particular limitation, may be set to generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %. It is recommended that the upper limit in the unsaturated carboxylic acid content (acid content), although not subject to any particular limitation, be generally not more than 25 wt %, preferably not more than 20 wt %, and more preferably not more than 15 wt %. At a low acid content, the rebound may decrease, whereas at a high acid content, the processability of the material may decrease.

The copolymer of component (a) accounts for a proportion of the overall base resin which is preferably from 95 to 50 wt %. The lower limit of this proportion is preferably at least 60 wt %, more preferably at least 70 wt %, and even more preferably at least 75 wt %. The upper limit is preferably not more than 92 wt %, more preferably not more than 89 wt %, and most preferably not more than 86 wt %.

The metal salt of the copolymer of component (a) may be obtained by neutralizing some of the acid groups in the random copolymer of component (a) with metal ions. Here, the metal ions which neutralize the acid groups are exemplified by Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In the present invention, of these, preferred use may be of Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ in particular, and Zn⁺⁺ is even more recommended. The degree of neutralization of the random copolymer by these metal ions, while not subject to any particular limitation, is generally at least 5 mol %, preferably at least 10 mol %, and especially at least 20 mol %. It is recommended that the upper limit in the degree of neutralization, while not subject to any particular limitation, be generally not more than 95 mol %, preferably not more than 90 mol %, and especially not more than 80 mol %. At a degree of neutralization in excess of 95 mol %, the moldability may decrease. On the other hand, at less than 5 mol %, it is necessary to increase the amount in which the inorganic metal compound serving as component (c) is added, which may present a drawback in terms of cost. Such a neutralization product may be obtained by a known method. For example, the neutralization product may be obtained by introducing a metal ion compound, such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide, into the random copolymer.

A commercial product may be used as component (a). Illustrative examples of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymers include those available under the trade names Nucrel AN4318, Nucrel AN4319, and Nucrel AN4311 (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of metal salts of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymers include those available under the trade names Himilan AM7316, Himilan AM7331, Himilan 1855 and Himilan 1856 (DuPont-Mitsui Polychemicals Co., Ltd.), and those available under the trade names Surlyn 6320 and Surlyn 8120 (E.I. DuPont de Nemours and Co., Ltd.).

Next, it is recommended that the weight-average molecular weight (Mw) of component (b) be preferably at least 100,000, more preferably at least 110,000, and even more preferably at least 120,000, and that the upper limit thereof be preferably not more than 200,000, more preferably not more than 190,000, and even more preferably not more than 170,000. The weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio for the copolymer is preferably at least 3, and more preferably at least 4, and the upper limit thereof is preferably not more than 7, and more preferably not more than 6.5.

Here, the olefin in component (b) is generally an olefin in which the number of carbons is at least 2 but not more than 8, and preferably not more than 6. Illustrative examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. The use of ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acid in component (b) include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

In addition, the random copolymer serving as component (b) may be obtained by the random copolymerization of the above ingredients in accordance with a known method. Here, the unsaturated carboxylic acid content (acid content) within the random copolymer, while not subject to any particular limitation, may be set to generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %. No particular limitation is imposed on the upper limit in the unsaturated carboxylic acid content (acid content), although it is recommended that this be generally not more than 25 wt %, preferably not more than 20 wt %, and more preferably not more than 15 wt %. At a low acid content, there is a possibility that the rebound will decrease, whereas at a high acid content, there is a possibility that the material processability will decrease.

In the above case, the copolymer of component (b) accounts for a proportion of the overall base resin which may be set to more than 0, and may be set to preferably at least 1 wt %. The upper limit, although not subject to any particular limitation, may be set to not more than 20 wt %, preferably not more than 17 wt %, more preferably not more than 10 wt %, even more preferably not more than 8 wt %, and most preferably not more than 5 wt %.

The metal salt of the copolymer of component (b) may be obtained by neutralizing some of the acid groups in the random copolymer of component (b) with metal ions. Here, preferred use may be made of, for example, Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ or Pb⁺⁺, as the metal ions which neutralize the acid groups. In the present invention, of these, more preferred use may be made of Na⁺, Li⁺⁺, Zn⁺⁺, Mg⁺⁺ or Ca⁺⁺. The use of Zn⁺⁺ is especially recommended. The degree of neutralization of the random copolymer by these metal ions, while not subject to any particular limitation, may be set to generally at least 5 mol %, preferably at least 10 mol %, and especially at least 20 mol %. It is recommended that the upper limit in the degree of neutralization, while not subject to any particular limitation, be set to generally not more than 95 mol %, preferably not more than 90 mol %, and especially not more than 80 mol %. At a degree of neutralization in excess of 95 mol %, the moldability may decrease. On the other hand, at less than 5 mol %, there arises a need to increase the amount in which the inorganic metal compound serving as component (c) is added, which may present a drawback in terms of cost. Such a neutralization product may be obtained by a known method. For example, the neutralization product may be obtained by introducing a metal ion compound, such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide, into the random copolymer.

A commercial product may be used as component (b). Illustrative examples include those available under the trade names Nucrel 1560, Nucrel 1525 and Nucrel 1035 (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of metal salts of the olefin-unsaturated carboxylic acid random copolymer include those available under the trade names Himilan 1605, Himilan 1601, Himilan 1557, Himilan 1705 and Himilan 1706 (DuPont-Mitsui Polychemicals Co., Ltd.), those available under the trade names Surlyn 7930 and Surlyn 7920 (E.I. DuPont de Nemours and Co., Ltd.), and those available under the trade names Escor 5100 and Escor 5200 (ExxonMobil Chemical).

Component (c) is a thermoplastic block copolymer composed of a crystalline polyolefin block and a polyethylene/butylene random copolymer. This component (c) is exemplified by thermoplastic block copolymers composed of a crystalline polyethylene block (E) as a hard segment and a block of a relatively random copolymer of ethylene and butylene (EB) as a soft segment. Preferred use may be made of block copolymers having a molecular structure with a hard segment at one or both ends, such as block copolymers having an E-EB or E-EB-E structure.

Such a component (c) may be obtained by hydrogenating a polybutadiene. Here, the polybutadiene used in hydrogenation is preferably one in which bonding within the butadiene structure is characterized by a 1,4-bond content in the butadiene structure as a whole of from 95 to 100 wt %, and in which from 50 to 100 wt %, and preferably from 80 to 100 wt %, of the 1,4-bonds are present as block-like regions.

The above-mentioned E-EB-E type thermoplastic block copolymer is preferably one obtained by hydrogenating a polybutadiene having at both ends of the molecular chain 1,4-polymerization products which are rich in 1,4-bonds and having an intermediate region where 1,4-bonds and 1,2-bonds are intermingled. The degree of hydrogenation (conversion of double bonds on the polybutadiene to saturated bonds) in the polybutadiene hydrogenate is preferably from 60 to 1000, and more preferably from 90 to 1000. Too low a degree of hydrogenation may give rise to undesirable effects such as gelation in the blending step with other components such as an ionomeric resin and, when the golf ball has been formed, may lead to a poor durability to impact.

In the block copolymer having an E-EB or E-EB-E molecular structure with a hard segment at one or both ends that may be advantageously used as the thermoplastic block copolymer, the content of the hard segments is preferably from 10 to 50 wt %. If the hard segment content is too high, the cover may lack sufficient softness, making it difficult to effectively achieve the objects of the invention. On the other hand, if the hard segment content is too low, the blend may have a poor moldability.

The thermoplastic block copolymer has a melt mass flow rate, at a test temperature of 230° C. and a test load of 21.2 N, of preferably from 0.01 to 15 g/10 min, and more preferably from 0.03 to 10 g/10 min. Outside of this range, problems such as weld lines, sink marks and short shots may arise during injection molding. Moreover, it is preferable for the thermoplastic block copolymer to have a surface hardness of from 10 to 50. If the surface hardness is too low, the golf ball may have a decreased durability to repeated impact. On the other hand, if the surface hardness is too high, a blend of the thermoplastic block copolymer with an ionomeric resin may have a decreased resilience. The thermoplastic block copolymer has a number-average molecular weight of preferably from 30,000 to 800,000.

A commercial product may be used as component (c). Illustrative examples include those available under the trade names Dynaron 6100P, Dynaron 6200P and Dynaron 6201B (JSR Corporation). Of these, Dynaron 6100P, which is a block polymer having crystalline olefin blocks at both ends, is especially preferred for use in the present invention. These olefinic thermoplastic elastomers may be used singly or as mixtures of two or more thereof.

In cases where component (c) is included in the resin component, the proportion of the total resin components accounted for by component (c) may be set to more than 0, and is preferably set to at least 5 wt %, more preferably at least 8 wt %, even more preferably at least 11 wt %, and most preferably at least 14 wt %. The upper limit, while not subject to any particular limitation, may be set to preferably not more than 50 wt %, more preferably not more than 40 wt %, even more preferably not more than 30 wt %, and most preferably not more than 20 wt %.

Component (d) is a fatty acid or fatty acid derivative having a molecular weight of at least 280 but not more than 1,500 whose purpose is to enhance the flow properties of the resin composition. It has a molecular weight which is very small compared with those of components (a) to (c), and helps to significantly decrease the melt viscosity of the mixture. Also, because the fatty acid (or fatty acid derivative) of component (d) has a molecular weight of at least 280 but not more than 1,500 and has a high content of acid groups (or derivative moieties thereof), its addition results in little loss of rebound.

The fatty acid or fatty acid derivative serving as component (d) may be an unsaturated fatty acid or fatty acid derivative having a double bond or triple bond in the alkyl moiety, or it may be a saturated fatty acid or fatty acid derivative in which all the bonds in the alkyl moiety are single bonds. It is recommended that the number of carbon atoms on the molecule be generally at least 18, with an upper limit of not more than 80, and especially not more than 40. Too few carbons may make it impossible to achieve an improved heat resistance and may also set the acid group content so high as to cause the acid groups to interact with acid groups present in the base resin, diminishing the flow-improving effects. On the other hand, too many carbons increases the molecular weight, as a result of which significant flow-improving effects may not appear, which may make the material difficult to use.

Specific examples of fatty acids that may be used as component (d) include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Of these, preferred use may be made of stearic acid, arachidic acid, behenic acid, lignoceric acid and oleic acid.

Fatty acid derivatives are exemplified by derivatives in which the proton on the acid group of the fatty acid has been substituted. Exemplary fatty acid derivatives of this type include metallic soaps in which the proton has been substituted with a metal ion. Metal ions that may be used in such metallic soaps include Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used as component (d) include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred.

The amount of component (d) included per 100 parts by weight of the resin component is at least 5 parts by weight, preferably at least 20 parts by weight, more preferably at least 50 parts by weight, and even more preferably at least 85 parts by weight. The upper limit in the amount included per 100 parts by weight of the resin component is not more than 170 parts by weight, preferably not more than 150 parts by weight, more preferably not more than 130 parts by weight, and even more preferably not more than 110 parts by weight.

Use may also be made of known metallic soap-modified ionomers (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Disclosure WO 98/46671) when using above components (a) and (b).

The basic inorganic metal compound of component (e) is included so as to neutralize acid groups in above component (a), component (d) and, if necessary, component (b). When above component (d) is not included, and in particular when a metal-modified ionomeric resin alone (e.g., a metal soap-modified ionomeric resin of the type mentioned in the foregoing patent publications, alone) is heated and mixed, as mentioned below, the metallic soap and unneutralized acid groups present on the ionomer undergo exchange reactions, generating a fatty acid. Because this fatty acid has a low thermal stability and readily vaporizes during molding, it causes molding defects. Moreover, if the fatty acid thus generated deposits on the surface of the molded material, it substantially lowers paint film adhesion.

(1) unneutralized acid group present on the ionomeric resin (2) metallic soap (3) fatty acid X: metal cation

To resolve such problems, a basic inorganic metal compound which neutralizes the acid groups present in above components (a), (b) and (d) is thus included as an essential component (component (e)). By adding component (e), the acid groups in above components (a), (b) and (d) are neutralized. Synergistic effects from the inclusion of these respective components increase the thermal stability of the resin composition while at the same time conferring a good moldability, and also impart the excellent property of enhancing rebound as a golf ball material.

It is recommended that component (e) be a basic inorganic metal compound—preferably a monoxide or hydroxide—which is capable of neutralizing acid groups in above components (a), (b) and (d). Because such compounds have a high reactivity with the ionomeric resin and the reaction by-products contain no organic matter, the degree of neutralization of the resin composition can be increased without a loss of thermal stability.

The metal ions used here in the basic inorganic metal compound are exemplified by Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Illustrative examples of the inorganic metal compound include basic inorganic fillers containing these metal ions, such as magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. Of these, as noted above, a monoxide or hydroxide is preferred. The use of magnesium oxide or calcium hydroxide, which have high reactivities with ionomer resins, is especially preferred in the present invention.

Component (e) is included in an amount, per 100 parts by weight of the resin component, of from 0.1 to 10 parts by weight. In this case, the lower limit is preferably at least 0.5 part by weight, more preferably at least 0.8 part by weight, and even more preferably at least 1 part by weight. The upper limit in the amount included per 100 parts by weight of the resin component is not more than 8 parts by weight, preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

The above-described resin composition which is obtained by blending components (a) to (e) can be provided with improved thermal stability, moldability and resilience. To this end, it is recommended that at least 70 mol %, preferably at least 80 mol %, and more preferably at least 90 mol %, of the acid groups in the resin composition be neutralized. A high degree of neutralization more reliably suppresses the exchange reactions that pose a problem in the above-described cases where components (a) and (b) and the fatty acid (or fatty acid derivative) alone are used, thus making it possible to prevent the generation of fatty acids. As a result, a material can be obtained which has a markedly increased thermal stability, a good moldability, and a substantially higher resilience than conventional ionomeric resins.

Here, with regard to neutralization of the above resin composition, to more reliably achieve both a high degree of neutralization and good flow properties, it is recommended that the acid groups in the resin composition be neutralized with transition metal ions and with alkali metal and/or alkaline earth metal ions. Because transition metal ions have a weaker ionic cohesion than alkali metal and alkaline earth metal ions, it is possible in this way to neutralize some of the acid groups in the resin composition and thus enable the flow properties to be significantly improved.

Various additives may also be optionally included in the above resin composition. Examples of additives which may be suitably included are pigments, dispersants, antioxidants, ultraviolet absorbers and optical stabilizers. Moreover, to further improve the feel of the golf ball on impact, the resin composition may also include various non-ionomeric thermoplastic elastomers. Illustrative examples of such non-ionomeric thermoplastic elastomers include styrene-based thermoplastic elastomers, ester-based thermoplastic elastomers and urethane-based thermoplastic elastomers. In this invention, the use of styrene-based thermoplastic elastomers is especially preferred.

Use may be made of a known mixing apparatus when preparing the above resin composition. For example, the above respective ingredients may be mixed using a twin-screw extruder, a Banbury mixer or a kneader. In such a case, the heating and mixing conditions may be suitably selected according to the type of material, and are not subject to any particular limitation. For example, mixing may be carried out at a temperature of from 150 to 250° C. The method of molding the cover using the above resin composition is also not subject to any particular limitation. For example, use may be made of an injection molding method or a compression molding method. When injection molding is employed, the process may involve placing a prefabricated core at a given position in the injection mold, then introducing the above material into the mold. When compression molding is employed, the process may involve producing a pair of half cups from the above material, covering the core with these half-cups, then applying pressure and heat within a mold. If molding under heat and pressure is carried out, the molding conditions used may be a temperature of from 120 to 170° C. and a period of from 1 to 5 minutes.

The cover material used in the invention may be a known cover material. Although not subject to any particular limitation, preferred use may be made of the above-described Polyurethane Material (I), Polyurethane Material (II) or an ionomeric resin material.

In the inventive golf ball, by combining dimples which satisfy the subsequently described specific parameters and are able to achieve a relatively low trajectory with an inner cover layer and an outer cover layer having the specific constructions described above, it is possible to greatly reduce the distance traveled by the golf ball on shots taken at a high head speed and also hold down the decrease in distance traveled on shots taken at a low head speed. The parameters for the dimples formed in the inventive golf ball are described in detail below.

In the present invention, dimples having the following parameters (1) to (10) are formed on the surface of the cover made of the above-described material. In cases where the surface of the ball is subjected to finishing treatment (e.g., painting and stamping) after the cover has been formed, parameters (1) to (10) below are calculated based on the shape of the dimples on the finished ball in which such treatment has been fully completed.

Dimple Parameter (1)

The total number of dimples is set in a range of at least 250 but not more than 500. The lower limit in the number of dimples may be set to preferably at least 280, more preferably at least 300, and even more preferably at least 340. The upper limit may be set to preferably not more than 450, more preferably not more than 420, and even more preferably not more than 400. In this range, the golf ball readily incurs lift, enabling the ball to travel farther, particularly on shots with a driver.

Dimple Parameter (2)

To improve aerodynamic performance, the dimple surface coverage (SR), defined as the sum of the surface areas on the surface of a hypothetical sphere that are circumscribed by the edges of the respective dimples as a proportion of the surface area of the hypothetical sphere, is set to at least 70%. SR may be set to preferably at least 71%, and more preferably at least 72%.

Dimple Parameter (3)

To improve the aerodynamic performance, the dimple volume ratio (VR), defined as the sum of the volumes of individual dimple spaces below a flat plane circumscribed by the edge of each dimple on a golf ball as a proportion of the volume of the golf ball were it to have no dimples on the surface (hypothetical sphere), is set to at least 1.0%. It is recommended that VR be set to preferably at least 1.1%, more preferably at least 1.15%, and even more preferably at least 1.2%. The upper limit is not more than 1.5%, preferably not more than 1.4%, and more preferably not more than 1.3%.

Dimple Parameter (4)

The number of dimple types, i.e., types of dimples of mutually differing diameter DM and/or depth DP, is set to three or more. The number of types may be set to preferably at least four, and more preferably at least five. The upper limit is preferably not more than 14 types, and more preferably not more than 10 types. The number of types of dimples is selected as appropriate in this way so as to facilitate an increase in the surface coverage SR specified in the invention.

Here, referring to FIG. 2, the depth DP of a dimple is the vertical distance from a hypothetical flat plane L, traced by connecting the positions where the dimple meets land areas, to the bottom (deepest position) of the dimple. In addition, as shown in FIG. 2, the diameter DM of a dimple is the diameter (span) between positions where the dimple portion is tangent with land areas (non-dimple forming portions), i.e., between the high points of the dimple portion. In most cases, the golf ball is subjected to surface treatment such as painting. In such balls, the dimple diameter and depth refer to the diameter and depth after the coat of paint has been applied.

Dimple Parameter (5)

To obtain a proper trajectory, the average dimple depth is set to at least about 0.18 mm. It is recommended that the average dimple depth be set to preferably at least about 0.19. The upper limit is preferably not more than about 1.0 mm, more preferably not more than about 0.7 mm, and even more preferably not more than about 0.5 mm. Here, “average dimple depth” refers to the average of the depths DP of all the dimples.

The average dimple diameter DM, while not subject to any particular limitation, is preferably at least about 3.0 mm, more preferably at least about 3.2 mm, and even more preferably at least about 3.5 mm. The upper limit is preferably not more than about 7.5 mm, more preferably not more than about 6.5 mm, and even more preferably not more than about 6 mm. Here, “average dimple diameter DM” refers to the average of the diameters of all the dimples.

Dimple Parameter (6)

The ratio of the dimple diameter DM to the dimple depth DP, or DM/DP, has an average value of not more than about 23. It is recommended that this average value be preferably not more than about 22, more preferably not more than about 21, and even more preferably not more than about 20. The lower limit, while not subject to any particular limitation, is preferably at least about 5, more preferably at least about 8, and even more preferably at least about 10.

Dimple Parameter (7)

In the present invention, although not subject to any particular limitation, when the dimples are divided into dimples Da having a diameter of 3.7 mm or more and smaller dimples Db, the (total number of Da dimples)/(total number of Db dimples) ratio is preferably set to at least about 0.005 but not more than about 1. The lower limit is more preferably at least about 0.01, even more preferably at least about 0.1, still more preferably at least about 0.2, and most preferably at least about 0.3. The upper limit is more preferably not more than about 0.8, even more preferably not more than about 0.6, and most preferably not more than about 0.5.

The dimples Da having a diameter of at least 3.7 mm account for a proportion of the total dimple volume which, while not subject to any particular limitation, is preferably at least about 75%, more preferably at least about 78%, and even more preferably at least about 80%. The upper limit value is preferably not more than about 98%, more preferably not more than about 95%, and even more preferably not more than about 92%.

The average diameter (Dm) of the Da dimples is preferably at least about 3.7 mm, and more preferably at least about 3.8 mm. The upper limit thereof is preferably not more than about 7 mm, and more preferably not more than about 6 mm. The average depth (Dp) of the Da dimples is preferably at least about 0.05 mm, and more preferably at least about 0.1 mm. The upper limit thereof is preferably not more than about 0.5 mm, and more preferably not more than about 0.3 mm. The average volume of the Da dimples is preferably at least about 0.8 mm³, and more preferably at least about 1.0 mm³. The upper limit thereof is preferably not more than about 3.0 mm³, and more preferably not more than about 2.5 mm³. The ratio Dm/Dp for the Da dimples is preferably at least about 7, and more preferably at least about 8, and the upper limit thereof is preferably not more than about 25, and more preferably not more than about 23. If the above numerical value ranges are not satisfied, the low trajectory that is desired may not be obtained, which may make it impossible to achieve the objects of the invention.

The average diameter (Dm) of the Db dimples is preferably at least about 1 mm, and more preferably at least about 2 mm. The upper limit is less than about 3.7 mm, and more preferably not more than about 3.5 mm. The average depth (Dp) of the Db dimples is preferably at least about 0.05 mm, and more preferably at least about 0.1 mm. The upper limit thereof is preferably not more than about 0.3 mm, and more preferably not more than about 0.2 mm. The average volume of the Db dimples is preferably at least about 0.2 mm³, and more preferably at least about 0.3 mm³. The upper limit thereof is preferably not more than about 1.5 mm³, and more preferably not more than about 1.0 mm³. The ratio Dm/Dp for the Db dimples is preferably at least about 10, and more preferably at least about 12. The upper limit thereof is preferably not more than about 30, and more preferably not more than about 26. If the above numerical value ranges are not satisfied, the low trajectory that is desired may not be obtained, which may make it impossible to achieve the objects of the invention.

Dimple Parameter (8)

To improve the distance a golf ball travels, it is desirable for the ball to have a low coefficient of drag (CD) under high-velocity conditions and a high coefficient of lift (CL) under low-velocity conditions. Thus, in the present invention, with regard to the low-velocity CL, it is critical for the coefficient of lift CL when the ball is launched using an Ultra Ball Launcher (UBL) at a Reynolds number of 70,000 and a spin rate of 2,000 rpm to be maintained at 60% or more, and preferably at 65% or more, of the coefficient of lift CL when the ball is launched at a Reynolds number of 80,000 and a spin rate of 2,000 rpm.

Dimple Parameter (9)

The dimples have an average edge angle of preferably at least 11 degrees, more preferably at least 12 degrees, and even more preferably at least 13 degrees. The upper limit is preferably not more than 17 degrees, more preferably not more than 16 degrees, and even more preferably not more than 15 degrees. If the average edge angle is too large, the trajectory may become too low, possibly resulting in too large a difference with a customary trajectory. On the other hand, if the average edge angle is too small, it may not be possible to obtain the effect of holding down the decrease in distance traveled by the ball on shots at a low HS while reducing the distance traveled on shots taken at a high HS. As used herein, “average edge angle” refers to the average edge angle for all the dimples.

Dimple Parameter (10)

It is recommended that the proportion of dimples having an edge angle of from 12 to 16 degrees be preferably more than 70%, more preferably at least 80%, and even more preferably at least 90%, of the total number of dimples formed on the ball surface. If the proportion of such dimples is too small, it may not be possible to obtain the effect of holding down the decrease in distance traveled by the ball on shots at a low HS while reducing the distance traveled on shots taken at a high HS.

The edge angle of a dimple is defined herein as follows. Referring to FIG. 9, let us imagine over the dimple D a first spherical surface (i.e., the spherical surface of the golf ball were it to have no dimples thereon) Q₁ prior to formation of the dimple. Let us also imagine a second spherical surface Q₂ which is centered at the center point of the golf ball and has a radius 0.04 mm smaller than that of the first spherical surface Q₁. If we then draw tangents T and T′ at points P and P′ where the second spherical surface Q₂ intersects the wall of the dimple D, the points E and E′ where the tangents T and T′ intersect the first spherical surface Q₁ represent the respective edges of the dimple D. The angle θ between the line segment (straight line) L connecting points E and E′ determined in this way and the tangents T and T′ is the edge angle.

The shapes of the dimples are not limited to circular shapes, and may also be suitably selected from among, for example, polygonal, tear-shaped and oval shapes. Setting the number of dimple types to at least three, and preferably at least five, makes it possible for the dimples to cover a higher proportion of the spherical surface. Also, by interspersing large and small dimples, the surface coverage can be increased to the specified range. Because this enables extreme fluctuations in the coefficient of lift (CL) within the low-velocity region to be suppressed, the ball can be given a relatively low trajectory, making it easier to elicit the advantageous effects of the invention.

The golf ball of the invention can be made to conform with the Rules of Golf for competitive play, and may be formed to a diameter of not less than 42.67 mm. It is suitable to set the weight to generally not less than 45.0 g, and preferably not less than 45.2 g, but not more than 45.93 g.

As described above, in this invention, it is possible to substantially reduce the distance traveled by the ball on high HS shots while at the same time holding down as much as possible the decrease in distance traveled on low HS shots. As a result, a superior golf ball for competitors having a low head speed can be obtained.

EXAMPLES

The following Examples and Comparative Examples are provided by way of illustration and not by way of limitation.

Examples 1 to 5, Comparative Examples 1 to 5

The rubber compositions shown in Table 1 were prepared, then molded and vulcanized at 155° C. for 15 minutes to produce solid cores. Numbers in the table indicate parts by weight.

TABLE 1 A B C D Formulation Polybutadiene 100 100 100 (parts by rubber (1) weight) Polybutadiene 100 rubber (2) Zinc acrylate 28.5 27.5 36.5 27.0 Peroxide (1) 0.3 0.3 1.1 0.3 Peroxide (2) 0.3 0.3 0.3 Zinc oxide 5 5 5 5 Barium sulfate 19.0 19.1 15.4 16.2 Calcium carbonate 5 Zinc stearate 5 5 5 5 Antioxidant 0.1 0.1 0.2 0.1 Zinc salt of 0.5 1 1 pentachloro- thiophenol Sulfur 0.09 Specific gravity 1.174 1.174 1.174 1.174

Trade names of the materials in the table are as follows.

-   Polybutadiene rubber (1): Available under the trade name “BR 730”     from JSR Corporation. -   Polybutadiene rubber (2): Available under the trade name “BR 01”     from JSR Corporation. -   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   Peroxide (1): Dicumyl peroxide, available under the trade name     “Percumyl D” from NOF Corporation. -   Peroxide (2): 1,1-Bis(t-butylperoxy)cyclohexane; available under the     trade name “Perhexa C-40” from NOF Corporation. -   Zinc oxide: Available from Sakai Chemical Industry Co., Ltd. -   Zinc stearate: Available under the trade name “Zinc Stearate G” from     NOF Corporation. -   Barium sulfate: Available under the trade name “Precipitated Barium     Sulfate 100” from Sakai Chemical Industry Co., Ltd. -   Calcium carbonate: Available under the trade name “Silver W” from     Shiraishi Calcium Kaisha, Ltd. -   Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi     Shinko Chemical Industry Co., Ltd.

Next, the cover material shown in Table 2 below was injection molded over the above core, thereby obtaining a multi-piece solid golf ball in which the core is encased by an inner cover layer and an outer cover layer of given thicknesses.

TABLE 2 1 2 Formulation Himilan 1557 15 (parts by weight) Himilan 1605 50 Himilan 1706 35 Pandex T8290 100 Trimethylolpropane 1 Polyisocyanate compound 9 Thermoplastic elastomer 15 Titanium oxide 3.5 Polyethylene wax 1.5

Trade names of the materials in the table are as follows.

-   Himilan: Ionomeric resins available from DuPont-Mitsui Polychemicals     Co., Ltd. -   Pandex T8290: A MDI-PTMG type thermoplastic polyurethane available     from DIC Bayer Polymer. -   Polyisocyanate compound: 4,4′-Diphenylmethane diisocyanate. -   Thermoplastic elastomer: Available under the trade name “Hytrel     4001” from DuPont-Toray Co., Ltd. -   Titanium oxide: Available under the trade name “Tipaque R550” from     Ishihara Sangyo Kaisha, Ltd. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd.

Numerous dimples were formed on the surface of the cover simultaneous with injection molding of the cover, after which the cover was spray-painted. In each example and comparative example, the dimples on the surface of the ball after painting satisfied the parameters shown in Tables 3 to 8 below. In these tables, the dimple types designated as Da refer to dimples having a diameter of 3.7 mm or more, and the dimple types designated as Db refer to dimples having a diameter of less than 3.7 mm.

With regard to the dimple patterns in the tables, the dimple pattern for Examples 1, 3 and 4 is shown in Table 3 (FIG. 3), the pattern for Examples 2 and 5 is shown in Table 4 (FIG. 4), the pattern for Comparative Examples 1 and 5 is shown in Table 5 (FIG. 5), the pattern for Comparative Example 2 is shown in Table 6 (FIG. 6), the pattern for Comparative Example 3 is shown in Table 7 (FIG. 7), and the pattern for Comparative Example 4 is shown in Table 8 (FIG. 8). These diagrams are all top views of the ball. In the respective examples, the bottom views of the ball have the same pattern as the top views, and are thus omitted.

TABLE 3 Examples Number Edge 1, 3 and 4 of Diameter Depth Volume angle Dimple types dimples (mm) (mm) (mm³) (°) Da-I 40 4.1 0.21 1.53 14.0 Da-II 184 3.9 0.20 1.31 14.0 Db-I 96 3.3 0.16 0.73 14.2 Da-III 32 4.1 0.23 1.72 15.6 Da-IV 16 3.9 0.22 1.45 15.4 Db-II 16 3.2 0.15 0.62 13.5 Db-III 8 3.2 0.14 0.49 11.3

TABLE 4 Examples Number Edge 2 and 5 of Diameter Depth Volume angle Dimple types dimples (mm) (mm) (mm³) (°) Da-I 24 4.5 0.20 1.66 12.3 Da-II 150 4.3 0.19 1.48 12.0 Da-III 66 3.7 0.18 1.02 12.3 Db-I 18 2.7 0.13 0.41 12.4 Db-II 6 2.5 0.12 0.31 12.2 Da-IV 48 4.3 0.19 1.56 13.7 Da-V 12 3.8 0.18 1.15 14.6 Db-III 6 3.4 0.16 0.75 11.8 Db-IV 6 3.3 0.15 0.66 11.4

TABLE 5 Comparative Examples Number Edge 1 and 5 of Diameter Depth Volume angle Dimple types dimples (mm) (mm) (mm³) (°) Da-I 24 4.7 0.15 1.25 9.8 Da-II 168 4.5 0.15 1.15 9.4 Da-III 48 3.9 0.15 0.85 10.3 Db-I 12 2.9 0.15 0.44 13.3 Db-II 12 2.6 0.11 0.24 11.6 Da-IV 30 4.4 0.16 1.20 10.2 Da-V 36 3.9 0.17 0.94 11.3 Db-III 8 3.5 0.16 0.70 11.8 Db-IV 6 3.4 0.15 0.61 11.4

TABLE 6 Comparative Number Edge Example 2 of Diameter Depth Volume angle Dimple types dimples (mm) (mm) (mm³) (°) Da-I 12 4.6 0.16 1.28 10.0 Da-II 222 4.4 0.16 1.16 9.9 Da-III 36 3.8 0.15 0.80 10.4 Db-I 12 2.6 0.12 0.58 12.0 Da-IV 12 4.4 0.17 0.25 10.6 Da-V 24 3.8 0.16 1.25 11.0 Db-II 6 3.5 0.16 0.86 11.8 Db-III 6 3.4 0.15 0.70 11.4

TABLE 7 Comparative Number Edge Example 3 of Diameter Depth Volume angle Dimple types dimples (mm) (mm) (mm³) (°) Da-I 228 4.3 0.17 1.06 11.5 Da-II 36 3.7 0.16 0.74 11.0 Db-I 12 2.5 0.12 0.23 12.2 Db-II 12 3.4 0.17 0.72 12.6 Da-III 42 4.3 0.18 1.14 12.2 Da-IV 24 3.7 0.17 0.80 11.8 Da-V 12 4.3 0.17 1.05 10.6 Da-VI 2 3.9 0.16 0.89 10.5

TABLE 8 Comparative Number Edge Example 4 of Diameter Depth Volume angle Dimple types dimples (mm) (mm) (mm³) (°) Db-I 114 3.65 0.20 1.07 13.3 Da-I 114 4.0 0.15 1.01 10.4 Db-II 60 3.65 0.20 1.07 13.3 Db-III 12 2.5 0.17 0.43 16.1 Da-II 60 4.0 0.15 1.01 10.4

Various properties of the resulting multi-piece solid golf balls were investigated as described below. The results are shown in Tables 9 and 10.

Center Hardness of Solid Core, Cross-Sectional Hardness Midway between Center and Surface of Core, and Surface Hardness of Core

The cross-sectional hardness (Shore D hardness) of the solid core was measured by cutting the core through the center, and perpendicularly pressing the indenter of a type D durometer conforming with ASTM D2240-95 against the center of the cross-section, and at a position midway between the center and surface of the cross-section.

The surface hardness (Shore D hardness) of the solid core was measured by perpendicularly pressing the indenter of a type D durometer conforming with ASTM D2240-95 against the spherical surface of the core.

The above hardnesses are each measured values obtained after holding the core isothermally at 23° C.

Deflection of Solid Core, Inner Cover Layer-Encased Sphere, and Ball

The solid core, inner cover layer-encased sphere and ball were placed on a hard plate, and the amount of deflection by each when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured. In addition, the deflection by the ball when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) was similarly measured.

The above deflections are each measured values obtained after holding the specimen to be measured isothermally at 23° C.

Cover Hardness (Shore D Hardness)

The cover-forming material was formed under applied pressure to a thickness of about 2 mm and the resulting sheet was held at 23° C. for 2 weeks, following which the Shore D hardness of the sheet was measured in accordance with ASTM D2240.

CL Ratio

The ratio of the coefficient of lift CL of a ball launched using an Ultra Ball Launcher (UBL) at a Reynolds number of 70,000 and a spin rate of 2,000 rpm with respect to the coefficient of lift CL of a ball launched at a Reynolds number of 80,000 and a spin rate of 2,000 rpm was calculated.

Initial Velocity

The initial velocity of the ball was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The ball was held isothermally in a 23±1° C. environment for at least 3 hours, then tested in a chamber at a room temperature of 23±2° C. The ball was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). One dozen balls were each hit four times. The time taken for the ball to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity (m/s) of the ball. This cycle was carried out over a period of about 15 minutes.

Flight Performance

A driver (W#1) was mounted on a swing robot, and the distance traveled by the ball when hit at a head speed (HS) of 54 m/s or 35 m/s was measured. The club used was a TOURSTAGE X-DRIVE 701 (2009 model; loft angle, 9.5°) manufactured by Bridgestone Sports Co., Ltd.

The flight performance was rated according to the following criteria.

-   -   Good: The difference in total distance between shots taken at a         head speed of 54 m/s and shots taken at a head speed of 35 m/s         was less than 97 m     -   NG: The difference in total distance between shots taken at a         head speed of 54 m/s and shots taken at a head speed of 35 m/s         was 97 m or more

TABLE 9 Example 1 2 3 4 5 Core Formulation A A B C C Diameter (mm) 37.3 37.3 37.3 37.3 37.3 10-130 kgf deflection (mm) 3.6 3.6 4.1 3.7 3.7 Center hardness Hc (Shore D) 42 42 37 36 36 Cross-sectional hardness Hm midway 47 47 42 40 40 between center and surface (Shore D) Surface hardness Hs (Shore D) 57 57 52 58 58 Inner cover Material 1 1 1 1 1 layer Material hardness (Shore D) 62 62 62 62 62 Thickness (mm) 1.7 1.7 1.7 1.7 1.7 Inner cover 10-130 kgf deflection (mm) 2.8 2.8 3.2 2.9 2.9 layer-encased sphere Outer cover Material 2 2 2 2 2 layer Material hardness (Shore D) 48 48 48 48 48 Thickness (mm) 1.0 1.0 1.0 1.0 1.0 Dimples Number of dimple types 7 types 9 types 7 types 7 types 9 types Number of dimples 392 336 392 392 336 SR value (%) 72 76 72 72 76 VR value (%) 1.20 1.06 1.20 1.20 1.06 Average DP (mm) 0.19 0.18 0.19 0.19 1.18 Average edge angle (°) 14.2 12.4 14.2 12.4 14.2 Proportion of dimples having an 98 96 98 98 96 edge angle of 12 to 16° (%) Average DM/DP 19.83 21.95 19.83 19.83 21.95 (Total number of Db dimples)/ 0.44 0.12 0.44 0.44 0.12 (Total number of Da dimples) Volume proportion of Da dimples (%) 82 96 82 82 96 Low-velocity CL ratio (%) 85 78 85 85 78 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 10-130 kgf deflection (mm) 2.6 2.6 3.0 2.6 2.6 10-600 kgf deflection (mm) 8.8 8.8 9.9 9.5 9.5 Initial velocity (m/s) 77.3 77.3 78.0 77.2 77.2 Hardness Hm − Hc (Shore D) 5 5 5 4 4 relationship Hs − Hm (Shore D) 10 10 10 18 18 Hs − Hc (Shore D) 15 15 15 22 22 Material hardness of inner cover 14 14 14 14 14 layer − Material hardness of outer cover layer (Shore D) Hs − Material hardness of −5 −5 −10 −4 −4 inner cover layer (Shore D) Hs − Material hardness of 9 9 4 10 10 outer cover layer (Shore D) Deflection Intermediate layer-encased sphere/Core 0.79 0.79 0.78 0.78 0.78 ratio Ball/Core 0.73 0.73 0.73 0.70 0.70 (600 kgf ball deflection)/(130 kgf 3.4 3.4 3.3 3.7 3.7 ball deflection) Flight HS, 54 m/s Carry (m) 263.5 265.9 262.1 263.4 266.4 Total distance (m) 276.5 278.0 276.0 277.2 279.0 HS, 35 m/s Carry (m) 165.2 165.6 164.9 165.0 165.7 Total distance (m) 183.6 183.1 183.7 184.2 183.8 Difference in carry (m) 98.3 100.3 97.2 98.4 100.7 Difference in total distance (m) 92.9 94.9 92.3 93.0 95.2 Rating good good good good good

TABLE 10 Comparative Example 1 2 3 4 5 Core Formulation D D D D A Diameter (mm) 37.3 37.3 37.3 37.3 37.3 10-130 kgf deflection (mm) 3.6 3.6 3.6 3.6 3.6 Center hardness Hc (Shore D) 42 42 42 42 42 Cross-sectional hardness Hm midway 47 47 47 47 47 between center and surface (Shore D) Surface hardness Hs (Shore D) 57 57 57 57 57 Inner cover Material 1 1 1 1 1 layer Material hardness (Shore D) 62 62 62 62 62 Thickness (mm) 1.7 1.7 1.7 1.7 1.7 Inner cover 10-130 kgf deflection (mm) 2.8 2.8 2.8 2.8 2.8 layer-encased sphere Outer cover Material 2 2 2 2 2 layer Material hardness (Shore D) 48 48 48 48 48 Thickness (mm) 1.0 1.0 1.0 1.0 1.0 Dimples Number of dimple types 9 types 8 types 8 types 5 types 9 types Number of dimples 344 330 368 360 344 SR value (%) 80 78 76 71 80 VR value (%) 0.90 0.88 0.93 0.90 0.90 Average DP (mm) 0.15 0.15 0.16 0.17 0.15 Average edge angle (°) 10.1 10.2 11.6 12.0 10.1 Proportion of dimples having an 3 4 18 52 3 edge angle of 12 to 16° (%) Average DM/DP 27.39 24.77 23.17 20.97 27.39 (Total number of Db dimples)/ 0.12 0.08 0.07 1.07 0.12 (Total number of Da dimples) Volume proportion of Da dimples (%) 95 95 97 48 95 Low-velocity CL ratio (%) 80 78 65 75 80 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 10-130 kgf deflection (mm) 2.6 2.6 2.6 2.6 2.6 10-600 kgf deflection (mm) 8.8 8.8 8.8 8.8 8.8 Initial velocity (m/s) 76.3 76.3 76.3 76.3 77.3 Hardness Hm − Hc (Shore D) 5 5 5 5 5 relationship Hs − Hm (Shore D) 10 10 10 10 10 Hs − Hc (Shore D) 15 15 15 15 15 Material hardness of inner cover 14 14 14 14 14 layer − Material hardness of outer cover layer (Shore D) Hs − Material hardness of −5 −5 −5 −5 −5 inner cover layer (Shore D) Hs − Material hardness of 9 9 9 9 9 outer cover layer (Shore D) Deflection Intermediate layer-encased sphere/Core 0.79 0.79 0.79 0.79 0.79 ratio Ball/Core 0.73 0.73 0.73 0.73 0.73 (600 kgf ball deflection)/(130 kgf 3.4 3.4 3.4 3.4 3.4 ball deflection) Flight HS, 54 m/s Carry (m) 267.1 267.5 265.9 266.5 272.2 Total distance (m) 277.0 276.6 276.3 276.0 283.9 HS, 35 m/s Carry (m) 161.2 162.0 162.6 162.2 165.2 Total distance (m) 177.7 176.9 178.0 177.3 182.2 Difference in carry (m) 105.9 105.5 103.3 104.3 107.0 Difference in total distance (m) 99.3 99.7 98.3 98.7 101.7 Rating NG NG NG NG NG

In the above table, Comparative Examples 1 to 4 are prior-art reduced-distance golf balls, and Comparative Example 5 is a prior-art high-rebound golf ball. Here, on comparing the reduced-distance golf balls of Examples 1 to 5 with those of Comparative Examples 1 to 4, it can be seen that the balls in Comparative Examples 1 to 4, owing to their lower rebound (initial velocity) relative to the prior-art high-rebound golf ball of Comparative Example 5, travel substantially reduced distances (both the carry and the total distance) not only at a high head speed but also at a low head speed. By contrast, it was confirmed that the golf balls in Examples 1 to 3 of the invention, by having the same rebound (initial velocity) as the high-rebound golf ball in Comparative Example 5 and by combining therewith dimples which satisfy specific parameters and can thus achieve a relatively low trajectory, are more effective than the balls in Comparative Examples 1 to 4 at suppressing the decrease in distance when hit at a low head speed relative to the substantial reduction in distance when hit at a high head speed. That is, the golf balls in the examples according to the present invention were confirmed to be golf balls which have a small difference in distance when hit at a high head speed versus when hit at a low head speed, and which are thus able to achieve a superior distance in the low head speed range while holding down the distance traveled in the high head speed range. 

1. A multi-piece solid golf ball comprising a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof, wherein the inner cover layer has a thickness of from 0.8 to 3.0 mm and a material hardness, in terms of Shore D hardness, of from 50 to 80, the outer cover layer has a material hardness, in terms of Shore D hardness, of from 35 to 60, and the material hardness of the inner cover layer is higher than the material hardness of the outer cover layer; the ratio of deflection by a sphere composed of the solid core encased by the inner cover layer (inner cover layer-encased sphere) when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (inner cover layer-encased sphere deflection)/(solid core deflection), is from 0.73 to 0.84; the dimples number at least 250 but not more than 500, have a surface coverage (SR) of at least 70% and a volume ratio (VR) of at least 1.0%, are of at least three types of mutually differing dimple diameter (DM) and/or dimple depth (DP), and have an average depth of at least about 0.18 mm and an average diameter-to-depth ratio (DM/DP) of not more than about 23; and the ball has a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is maintained at 600 or more of a coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm.
 2. The multi-piece solid golf ball of claim 1 wherein, letting Da represent dimples having a diameter of at least 3.7 mm and Db represent dimples having a diameter of less than 3.7 mm, the ratio (total number of Db dimples)/(total number of Da dimples) is at least about 0.005 but not more than about
 1. 3. The multi-piece solid golf ball of claim 2, wherein the dimples Da having a diameter of at least 3.7 mm account for at least about 75% of the total dimple volume.
 4. The multi-piece solid golf ball of claim 1, wherein the outer cover layer has a thickness of from 0.5 to 3.0 mm.
 5. The multi-piece solid golf ball of claim 1, wherein the value obtained by subtracting the material hardness of the outer cover layer from the material hardness of the inner cover layer (inner cover layer material hardness−outer cover layer material hardness) is, in terms of Shore D hardness, at least 5 but not more than
 50. 6. The multi-piece solid golf ball of claim 1, wherein the dimples have an average edge angle of from 11 to 17 degrees.
 7. The multi-piece solid golf ball of claim 1, wherein the proportion of dimples having an edge angle of from 12 to 16 degrees is more than 700 of the total number of dimples formed on the surface of the ball.
 8. The multi-piece solid golf ball of claim 1, wherein the value obtained by subtracting the inner cover layer material hardness from a surface hardness (Hs) of the core (Hs—inner cover layer material hardness) is, in terms of Shore D hardness, at least −15 and not higher than +5.
 9. The multi-piece solid golf ball of claim 1, wherein the value obtained by subtracting the outer cover layer material hardness from a surface hardness (Hs) of the core (Hs—outer cover layer material hardness) is, in terms of Shore D hardness, at least 0 and not higher than +20.
 10. The multi-piece solid golf ball of claim 1, wherein the ratio of deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (ball deflection)/(solid core deflection), is from 0.67 to 0.76.
 11. The multi-piece solid golf ball of claim 1, wherein the ratio of deflection by the ball when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) to deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (600 kgf deflection/130 kgf deflection), is from 3.3 to 3.8.
 12. The multi-piece solid golf ball of claim 1, wherein the core has a center hardness (Hc), a surface hardness (Hs) and a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, in terms of Shore D hardnesses, satisfy the following conditions: Hm−Hc=0 to 7, Hs−Hm=11 to 25, and Hs−Hc≧16.
 13. The multi-piece solid golf ball of claim 1, wherein the core has a center hardness (Hc), a surface hardness (Hs) and a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, in terms of Shore D hardnesses, satisfy the following condition: Hs−Hm>(Hm−Hc)×3.
 14. The multi-piece solid golf ball of claim 1 wherein dimples Da with a diameter of at least 3.7 mm have an average diameter of at least 3.7 mm but not more than 6 mm, and dimples Db with a diameter of less than 3.7 mm have an average diameter of at least 1 mm but less than 3.7 mm.
 15. The multi-piece solid golf ball of claim 1, wherein dimples Da with a diameter of at least 3.7 mm have an average depth of from 0.05 to 0.5 mm, and dimples Db with a diameter of less than 3.7 mm have an average depth of from 0.05 to 0.3 mm.
 16. The multi-piece solid golf ball of claim 1, wherein dimples Da with a diameter of at least 3.7 mm have an average volume of from 0.8 to 3.0 mm³, and dimples Db with a diameter of less than 3.7 mm have an average volume of from 0.2 to 1.5 min³.
 17. The multi-piece solid golf ball of claim 1, wherein dimples Da with a diameter of at least 3.7 mm have an average diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 7 to 25, and dimples Db with a diameter of less than 3.7 mm have an average diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 10 to
 30. 18. The multi-piece solid golf ball of claim 1, wherein the cover is formed of a material comprising: (A) a thermoplastic polyurethane material, and (B) an isocyanate mixture obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate.
 19. The multi-piece solid golf ball of claim 1, wherein the cover is formed of a material comprising: (D) a thermoplastic polyurethane, and (E) a polyisocyanate compound. 